Hotjse electric



' as found in nature, have .tals of different sizes.

Patented Mar. 8, 1932 UNITED STATES PATENT OFFICE JOSEPH AIME LAVALLEE,or srmenmb MASSACHUSETTS, ASSIGNOR TO WESTING- VAN IA HEAT TREATMENT OFPIEZO-ELECTRIC-CIIYSTAL SECTIONS No Drawing.

My invention relates to frequency-control devices and it has particularrelationto the manufacture of tions.

Quartz, by reason of its mechanical strength, is the crystallinesubstance that is,

most generally, used for controlling, through itspiezo-electriccharacteristics, the frequency of oscillation generators comprisingthermionic tubes. The thickness of a quartz plate determines theoscillation frequency thereof; the thinner the plate the hi her thefrequency. A plate having a thicl ness of substantially three incheswill oscillate at the rate of about 30,000 cycles per secondya plate .5millimeter in thickness will oscillate at a frequency of about 3,500,000cycles per second. I n making quartz-crystal plates for the control ofoscillation generators at high frequencies, great difficulty has,heretofore, been experienced in obtaining plates that would resonate atsubstantially a single frequency. The phenomenon of plural-resonanceappears to be caused by lack of absolute arallelism between thecrystal-section faces -t at lie perpendicular to the dimension whichdetermines the oscillation frequency.

The percentage of deviation from parallelism, in thin plates is,obviously, much greater than in thick plates,

words, in a thick plate .the number of deviations from a plane surace,-as compared with the mass of the plate, is vastly less than when athin plate is considered. v

It is highly probable thatquartz crystals,

' grown by the apposition of an infinite number of microscopic crys-'The ratio of the total volume of the microscopic crystals to the totalof the inter-crystal spaces is not known to me, at this time, but itseems reasonably certain that a quartz crystal has, actually, a veryloose or lattice-like structure and that the constituent microscopiccrystals have a certain freedom of motion.

- It is, accordin 1y, reasonable to assume that the ratio of the massinvolved in the deviations from a plane surface to the mass of theentire crystal plate can be changed by lpiezo-electric crystal sec--provided that equal care ls used in the manufacture of both or, in otherApplication filed December 5, 1923). Serial 1T0. 411,999.

changing the temperature of the plate. Or, to state the matter somewhatdifl'erently, heating causes such molecular change, or movement, withinthe crystal that the deviations are accentuated.

Irres ective of the correctness of the foreoing tiieor however, it is anexperimental act that, w en thin quartz plates are heated to aboutcentigrade the oscillations thereof are materially. damped, with the,consequent production of harmonic frequencies and other fre uencies thatare undesirable. Thick latesfhowever, are not greatly affected y heatingto the same temperature, probably because the percentage of deviationsto mass is so much lower than in thin crystals.

-It is, accordingly, an object of my invention to provide, as an articleof manufacture, a quartz plate that shall be uni-resonant, asdistinguished from plural-resonant plates of the prior art.

Another object of my invention is to pro- I 'vide a -manufacturingmethod for quartzcrystal plates that shall ensure the production ofplates which are not detrimentally affected by being operated attemperatures of the order of 50? ceutigrade.

A still further object of my invention is to provide a manufacturingmethod for quartz-crystal plates that shall ensure the production ofplates which resonate at substantially a single frequency.

In practicing my invention, I first cause acrystal section to be cutfrom a mothercrystal, preferably in the manner disclosed in thecopending application of D. G. Little, Serial Number 187 ,125, filedApril 28, 1927 and assigned to the Westinghouse Electric andManufacturing Company. The exact manner of cutting the section from thecrystal is not determinative, however, since my invention may, bepracticed in connection with crystals cut in any of the well-known ways.

with equally as good results,

I next give the crystal section a preliminary rough grinding to bring itto approximately the size that experience has shown 1s correct'for thefrequency desired, A final grinding and polishing is then given, with.frequent comparisons with a standard oscillator, until the sectionresonates at a frequency only slightly lower than that desired. Thecrystal is then placed in a sand-bath,

and the temperature is gradually-raised to between 300 and 1000Fahrenheit and main tained thereat for a period of from 3 to 5.

' hours; The table below gives the approxi mate temperatures andtreatment periodsfo'r crystals of different thickness and is indicativeof the proper procedure to follow:

Thickness Frequency g gg t figg 2 In. m. 1,000 K. C. 500 F. 5 hrs.

1.30 m. m. 1,500 K. 0. 500 F. 5 hrs. 1 m. m. 2,000 K. 0. 500 F. 5 hrs.

The crystal, after the heating has been maintained for the necessaryperiod, is neat allowed to cool slowly, in the sand-bath, until itreaches room temperature and may be l as safely handled. Sudden coolingshould be avoided because the crystal, if very thin, is liable tobreakage thereby.

A final polishing is then given the section, with frequent tests, untilsuflicient material has been removed to cause it to resonate, within thepermissible degree of error, to the predetermined frequency.

It will be seen, from a consideration of the foregoing description of mymethod of crystal-treatment, that it is somewhat analogous to theanneallng of metal. It is my present belief that the heat treatmenteither removes internal strains in the crystal or else causes 7 themolecular structure to so change that subsequent heating, duringoperation, does not cause an accentuation of the percentage of deviationfrom parallelism of the faces, The matter of theory, howevenisrelatively un important since, regardless of the type of change causedby the heat treatment, it is found that crystals so treated are d funi-resonant and are not deer damped by molecular changes durtemperatures higher than room A crystal section made ace g improvedmethod is, also, less liable l c age during use than untreated sectio s,1 further justifying the time expense involved in the treatment,

Although I have explained my invention in connection with a specificmethod of heattreatment many modifications thereof will be apparenttothose skilled in the art. My invention, therefore, is not to berestricted except insofar as is'necessitated by the prior art and by thespirit of the appended claims.

I claim as my invention:

1. As an article of manufacture, a quartzcrystal section that has beenannealed, at a' temperature between 300 and 1000 Fahrenheit.

2. As an article of manufacture, a quartzcrystal section devoid ofinternal molecular strains.

3. The method of manufacturing a quartzcrystal section which comprisesshaping the section to approximately correct dimensions and thereaftermaintaining it at an elevated temperature for a suilicient length oftime to relieve any internal stresses that may exist in it. a

4. The method of manufacturing a quartzcrystal section which comprisesshaping the section to approximately correct dimensions and thereafterraising its temperature to between 3009 and 1000 Fahrenheit andmaintaining it at the raised temperature a sufficient length of time tocause a readjustment of the molecular structure-thereof.

In testimony whereof I have hereunto subscribed my name this thirtiethday of November, 1929.

r J. AIME LAVALLEE.

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